1. Field of the Invention
This invention relates to a method of controlling a display screen of a terminal unit of a data processing system, or more in particular, to a display control method for a multi-window system in which a plurality of rectangular display regions called windows are set in a display screen for independent data display in each window.
2. Description of the Prior Art
In a data processing terminal having a display unit, a retangular region called a "window" is set on the display screen, and data representing such as a document and read-out from a data file unit is displayed in this window. By key input operation while referring to this window another document is displayed in another region on the display screen. In what is called a multi-window system with a plurality of windows set on a single display screen, the windows are set in partially overlapped relationship with each other. By shifting the window display position or changing the order in preference of display, the display in a given window which has been initially hidden in part behind another window can be made wholely visible.
In the multi-window system, the respective windows set on the display screen correspond to logical or virtual display screens set on a memory which is referred to by a data processing system. Data stored in a part of this virtual screen is extracted, and developed as a bit map in a predetermined region on a frame memory corresponding to an actual display screen so as to be read-out as the contents to be displayed in the window.
Data representing the position and size of each window on the actual screen and the correspondence between the window and an extraction region on the virtual screen are registered in a specific area alloted to that window on a window control table, so that the operator can, by inputting a screen operation command into the data processing system, change and control the contents of the display in any desired window on the display screen by reference to the window control table.
Assume, for example, that the operator designates a specific one window by cursor and gives a command to shift this window to another position on the screen designated by cursor. On the window control table, the positional data of the particular window is rewritten and the contents of the frame memory are changed, thereby shifting the window position on the display screen. When a window is specified and a command to change the size thereof is given, on the other hand, the window size data on the window control table is rewritten, and the contents of the frame memory are changed so that a window of a different size appears on the display screen. Further, if a command is given to scroll the contents of display in a window, the position of a partial region on the virtual screen from which the data to be displayed is extracted is shifted so thst an image corresponding to the shifted region is displayed in the window.
In conventional multi-window systems, the position of a window is changed in such a manner that after the original window is cancelled from the screen, data is again extracted from a partial region on the virtual memory corresponding to the new window, and developed as a bit map at the new window position or the frame memory. In changing the window size, too, data is extracted from a partial region on the virtual memory corresponding to a new window size and developed as a bit map on the frame memory. This is also the case with the scrolling process, in which all the data required for display after scrolling are extracted from the virtual screen and developed as a bit map on the frame memory. The process of developing display data into a bit map such as converting data stored in the form of character code on the virtual screen into the form of character font, however, takes considerable time, and therefore a conventional method in which all the contents to be newly displayed must be developed into a bit map has a problem that it cannot rapidly replace the contents on the display screen.
In a multi-window system where a plurality of windows are set in a partially overlapped relationship with each other on the display screen, on the other hand, the data for a window lower in display order extracted from the virtual screen and supplied to the frame memory must exclude those data corresponding to the invisible part thereof overlapped with another window higher in display order. For the purpose of such partial display or non-display in each window as mentioned above, Japanese Patent Publication No. JP-A-59-102284 (1984) discloses a control system in which each window is divided into a plurality of rectangular subregions along the sides of other windows overlapped therewith as dividing lines, the data representing the position and size of each subregion together with the data discriminating each subregion "visual" (displayable) or "non-visual" (non-displayable) are stored in a visible region control table, and display data are applied only to the displayable subregions.
In the conventional multi-window systems, the above-mentioned data set on the display screen for visible and non-visible subregions on each of all the windows are always registered in the visible region control table, and in the case of a change in the display order or positional relationship of the windows or addition of another window on the screen, the data in the visible region control table are updated in accordance with the new arrangement in display of the display screen, thus making it possible to change the display of any selected window immediately by referring to the control table. With the complication of superimposition of windows on the display screen, however, windows are much divided into smaller subregions, resulting in that the conventional methods in which the data for visible subregions of all the windows must be always stored require a large memory capacity of the visible region control table, as well as a problem of an increased processing time for updating the data of the table.